1. Field of the Invention
The invention relates to a liquid-cooled casting die for a continuous casting installation having a form-giving casting die body made of a material of high thermal conductivity, such as copper or a copper alloy.
2. Description of Related Art
Casting dies are designed to remove heat from the molten metal and to make it possible for the billet to solidify all the way through, beyond the casting shell that forms at the outset.
Various casting die geometries are in use, depending on the application, such as casting die tubes in round, rectangular, or complex shapes. Casting die plates are used for square/rectangular cogs [cogged ingots] or for slabs having greater height-width ratios. In addition, there are special geometries, such as preliminary sections for double-T supports and thin-slab casting dies having funnel expansion in the upper plate area for receiving the pouring nozzle. It is characteristic of all these casting dies that their goal is a homogeneous cooling of the surfaces. The corner areas represent special cases since in plate-type casting dies, by virtue of the design, there are, for example, abutting edges having disrupted cooling. In addition, there are some areas having larger material volumes for the reverse-side mounting elements, the areas, with a view to identical cooling, being adjusted at the start using specially configured groove-shaped coolant channels.
It is also known to provide improved cooling to casting dies subject to particularly high thermal stresses, in order to avoid premature damage to the casting die. This means in the case of thin-slab casting dies, that the thermal resistance of the casting die wall should not be too great, for which reason thinner walls are chosen. Moreover, given the higher pouring rates that are targeted, particular demands are placed on cooling-water quality and flow rate.
All of the cited measures have the same goal, to provide the pouring side of the casting die body with the best possible, homogeneous cooling. Potential areas of disruption due to the type of construction, such as at reverse-side cooling surfaces, are eliminated when the occasion arises, in order to obtain once again a uniform cooling.
The local conditions of stress in the use of funnel casting die plates are dependent on the operating conditions. On the pouring side, they are basically determined by the kind of steel pouring temperature, the speed, the lubrication/cooling conditions of the pouring powder, the geometry of the pouring nozzle, and the corresponding flow of the molten mass. On the other side, the water side, the casting die temperatures are determined by the quality, quantity, and flow rate of the cooling water. These variables are partly determined already by the casting die design, such as in the geometry of the coolant channels.
Using the destructive test of numerous casting die plates in use in various steel mills, however, the actual stressing and also the damage resulting thereby of the casting die material can be clearly determined. On the basis of these tests, a varying weakening of the surface and of the area near the surface extending across the width of the meniscus can be established.
Thus, in the critical area, the hardness falls from 100% of the output value to approximately 60%, whereas at the same level near the critical area, only a fall of approximately 70% of the output hardness is measured; in this context, the edge area of the casting die plates does not come into consideration. Similar results are yielded by measurements of the wall thickness after use of the casting die plates; identical material weaknesses in the critical area of the bath surface extending across roughly one-third of the greater depths in comparison to the uncritical areas.
Thin-ingot casting dies are stressed to different extents as a result of the varying influences on the broad side walls. Among these influences are essentially:                a high flow rate of the steel molten mass; turbulence of the molten mass particularly stresses the transitional areas of the funnel into the plane-parallel sides of the casting cross-section.        a higher mechanical stressing of the wall of the copper plate bent in the funnel discharge as a result of thermal expansion. The resulting stresses are particularly high on the pouring side.        
This leads to a particularly pronounced softening of the casting die material in this transitional area of the funnel. As a result of the locally relatively higher temperatures and the higher material loads related to the respective resistance to heat of a material-volume element, cracks can appear prematurely in this surface area. These cracks are more likely to occur due to a diffusion process, marked here as temperature dependent, of Zn-atoms from the steel into the Cu-matrix, because the Cu—Zn phases which arise form a hard and brittle surface layer which makes possible higher rate of crack formation.